In Kastigar v. United States, the Supreme Court held that in order to compel testimony containing self-incriminating information from a witness (1) who is not a criminal defendant and (2) has invoked the privilege against self-incrimination, the government must grant the witness immunity from “use and derivative use” of the testimony in a subsequent criminal proceeding against him. The federal circuits are split on the scope of protection afforded by this type of immunity. Some circuits construe use and derivative use immunity to prohibit any and all uses the government may make of his immunized testimony. Other circuits maintain that the immunity bans only evidentiary uses, and therefore permit all nonevidentiary uses. In the 2011 case United States v. Slough, the D.C. Circuit joined the latter group and held that a prosecutor may use compelled testimony as the basis of her decision to prosecute the witness who gave it. The court reasoned that to hold otherwise would unduly infringe on prosecutorial discretion.
This paper proposes a novel interpretation of use and derivative use immunity that falls in between both sides of the circuit split. Based on the language of Kastigar and policy considerations, the protection afforded by use and derivative use immunity is substantially equivalent to the protection a witness would receive if he had been able to exercise the right to remain silent. It follows that an immunized witness should be protected from any harmful use of his testimony by the government. Using a witness’s testimony to decide to prosecute him constitutes a harmful use. Additionally, while prosecutorial discretion is broad, a defendant may not be prosecuted based on the exercise of his constitutional or statutory rights. Giving compelled testimony requires constitutional protection commensurate with exercising the right to remain silent. So prosecuting an immunized witness based on his compelled testimony is equivalent to prosecuting him for exercising his right to remain silent. This the government may not do. Therefore, Slough was wrongly decided.